Kotaro dosent need a harem, he has chizu-nee the no.1 in the class. Not to mention Natsumi...
Negi should never run out of pills, because this is fantasy where logic does not apply.
Should he run out of them, he can buy more from the local mage market.
The real problem is explaining the magic and Age Pills situation to the non-magic group, especially with Ako, since Ako loves "Nagi" who is actually Negi.... It would be hella funny if Negi has to keep up with his "Nagi" diguise to fool Ako. On the other hand, it would be just as amusing to see Ako's reaction of learning the truth, Nagi--------->Negi.